[Nerfix]

Yes, yes you should start using def. Pythium. The Cult is a lovely theme and it oozes with hydra flavour but it really just doesn't cut it.

As for regular Pythium...never played them that much but here goes:

Masses of weak undead are easy peasy since you have priests with Water magic. Research Quickness and spam Banishment with them.

You also have a pretty easy acces to Astral battle magic, namle Soul Slay and Enslave and Air battle magic like Thunder Strike and Orb Lightning. OL is wonderful since it scales in a sweet, sweet manner with your air magic level but early on TS and even regular Lightning Bolt is very good.

Don't forget Quickness. Use it. Feel the power of shooting two spells per round with a quickened caster.

Don't forget communion either. Recruit Communicants or those 1 S Theurgs. Communicants have no magic skills but they have Communion Slave autocast on them at all times. The S1 Theurgs need to use the spell Communion Slave but can also do something useful on the field. Well, semi-useful.

Communion allows you to increase the power of mages who have cast Communion Master by 1 for every two mages who have cast Communion Slave. The fatigue from the master's spells partially goes to the Slaves, but the buff spells are also transfered to them. So if you cast Summon Earthpower and Bless on a Master the Slaves will get the Earthpower effect and Bless if they are sacred. This, as you can imagine, is very useful.

You can easily rack up amazingly high Holy magic levels and magic levels in general while easening the fatigue burden. But keep in mind that the fatique can quickly become deadly for the slaves. A pro way the fight fatigue is the have a Nature Theurg as one of the Masters and then have him cast Relief.

A more demo friendly way is taking enough Earth magic to get some of the Earth bless and then have an Earth theurg be a master and have him cast Summon Earthpower and Bless. This will help a bit with the fatigue.

Setting up a big Communion with multiple Slaves (some have used up to 16 or so slaves) and with several Masters can be difficult but it pays off.

Emerald Guards are very good units but they are too resource intensive for start in my opinion. Regular troops (I believe Princes are where it's at) with Standards mixed in are good enough for start I believe. Support them with mages and summons. Emerald Guards become useful if/when you enough resources to build them in proper numbers.